BEHAVIOR OF THE SM-ND, RB-SR, K-AR, AND U-PB ISOTOPIC SYSTEMS DURING ALKALINE METASOMATISM: FENITES IN THE OUTER-CONTACT ZONE OF AN ULTRAMAFIC-ALKALINE INTRUSION

Abstract

The behavior of isotopic systems (Sm-Nd, Rb-Sr, K-Ar, and U-Pb) in minerals from Archean plagiogneisses in the outer-contact zone of an alkaline-ultramafic intrusion points to an age discordance that resulted from the active action of the alkaline CO2-bearing fluid. The K-Ar system of biotite is the least susceptible to the chemical influence of the fluid, with the age of this mineral becoming systematically older away from the contact according to variations in the thermal field of the intrusion. At the same time, closer to the contact, the potassic feldspar loses not only 40Ar but K also, whose concentration increases by one order of magnitude (from 0.3 to 2.7%) in the plagioclase. The latter mineral also becomes richer in volatile components (40Ar and CO2) in the fenitization zone. The influence of the fluid on zircon is manifested in Pb and U migration and results in a newly formed metasomatic population of this mineral. The maximum U-Pb discordance arises inside zircon grains, with preservation of their thin zones of Archean age. The data obtained on the Sm-Nd and Rb-Sr systems definitely indicate that Sr and Nd were transported from the intrusion into the host gneisses and that the transport kinetics of these elements in the pore (interstitial) space was different. The character of changes in the isotopic composition of rocks during metasomatism differs from simple mixing and corresponds to the ion exchange mechanism in the rock-fluid system, when the transport of elements in fluid along an array of fractures is associated with the diffusion of these elements in the interstitial space of the rock. Thereby Sr quickly achieves equilibrium with the host rocks during the formation of metasomatic minerals in them, while Nd can be transported for long distances (in the form of soluble carbonate complex) via fractures and veinlets and is accommodated in the minerals of the vein assemblage (apatite, sphene, and pyrochlore).